[clangd] Don't attach FixIt to the source code in macro.

Summary:
We are less certain it is the correct fix. Also, clang doesn't apply FixIt to
the source code in macro.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58525

llvm-svn: 354664